1. Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular to improved filter fibers for use in Raman lasing applications and techniques for designing and manufacturing such fibers.
2. Background Art
Fiber lasers and amplifiers are typically based on optical fibers that are doped with laser-active rare earth ions, such as ytterbium (Yb), erbium (Er), neodymium (Nd), and the like. Stimulated Raman scattering in optical fibers is a useful effect that can be employed in order to provide nonlinear gain at wavelength regions in which these rare earth doped fibers do not operate. Stimulated Raman scattering occurs when a laser beam propagates through a Raman-active fiber, resulting in a predictable increase in wavelength, known as a “Stokes shift.” By providing a series of wavelength-specific reflector gratings at the input and output ends of a length of a Raman-active fiber, it is possible to create a cascaded series of Stokes shifts in order to convert an input wavelength to a selected target wavelength.
FIG. 1 is a diagram of an exemplary system 20 according to the prior art, in which stimulated Raman scattering is used to generate a high-power output 80 at 1480 nm for pumping an erbium-doped fiber amplifier (EDFA), which provides gain in the 1550 nm region. As illustrated, the system 20 comprises two stages: a monolithic Yb-fiber laser 40 and a cascaded Raman resonator (CRR) 60.
In laser 40, the active medium is provided by a length of a double-clad Yb-doped fiber 42 operating in the region of 1000 nm to 1200 nm. A high reflector grating HR1 is provided at the fiber input end 44, and an output coupler grating OC1 is provided at the fiber output end 46. The portion of fiber 42 between the high reflector HR1 and the output coupler OC1 functions as a laser cavity 48. Pumping energy is provided to fiber 42 by a plurality of pump diodes 50, which are coupled to fiber 42 by means of a tapered fiber bundle TFB1. In the present example, laser 40 provides as an output 52 single-mode radiation at a wavelength of 1117 nm.
The laser output is used to pump the cascaded Raman resonator 60. Resonator 60 comprises a Raman-active fiber 62. A plurality of input gratings 64 are provided at the fiber's input end 66, and a plurality of output gratings 68 are provided at the fiber's output end 70. The plurality of input gratings 64 includes high reflectors HR2-HR6; the plurality of output gratings 68 includes high reflectors HR7-HR11 and output coupler OC2.
Exemplary wavelengths ranging from 1175 nm to 1480 nm are shown for the input high reflectors HR2-HR6, output high reflectors HR7-HR11, and output coupler OC2. As shown in FIG. 1, the input gratings 64 and the output gratings 68 include a nested series of wavelength-matched pairs, separated by a respective Stokes shift. The input gratings 64, output gratings 68, and Raman fiber 62, provides a nested series of Raman cavities 72. While FIG. 1 shows cascaded Raman resonator 60 constructed using gratings 64 and 68, it is well known that similar resonators may be constructed using other wavelength-selective elements, such as fused-fiber couplers, and other architectures, such as WDM loop mirrors.
The 1117 nm output 52 of the Yb-doped fiber laser 40 is launched as an input into the resonator 60, resulting in a cascaded series of Stokes shifts over a broad range, resulting in a stepwise increase in wavelength from the 1117 nm input to a 1480 nm system output 80. One application of the output 80 can then be used to pump a high-power, silica-based erbium-doped fiber amplifier (EDFA) in the fundamental mode, which provides gain in the 1530 to 1590 nm region.
However, in system 20, a certain amount of Raman scattering continues to occur even after the target wavelength has been achieved. Thus, at higher powers, a significant amount of pumping energy may be lost because of light being transferred to the next, unwanted, higher-order Stokes shift. This unwanted Stokes shift limits the amount of power that can be obtained at the desired output wavelength. Furthermore, if the output 80 of the CRR is used to pump an EDFA, the unwanted higher-order Stokes shift can potentially interfere with signal wavelengths being amplified in the EDFA.